The details of how you start creating a station are not known yet, but I think it will likely involve filling out a holographically projected blueprint with a construction tool, either ship- or hand-mounted. Stations will need to have players around to simulate them, if nobody is around, all activity within the station will cease until somebody comes within simulation range again. The non-personal weapons in the game are one size only right now, although you might be able to build much heavier turrets than the ones you can put on ships - ones mounted on rails are reportedly unstable on ships right now, but that has to do with the game physics related to ship movement so they'll probably work fine on stations.